1. Field of the Invention
This invention relates to a method and apparatus for manufacturing a bellows pipe, particularly of small diameter.
2. Description of the Prior Art
In the field of a method and apparatus for manufacturing a bellows pipe, there are known Japanese Patent Publication No. 44-24746 (Bulge machining apparatus), Japanese Patent Laid-Open No. 59-133021 (Method and apparatus for manufacturing corded pipe), Japanese Patent Laid-Open No. 63-207421 (Method and apparatus for manufacturing beaded pipe), etc.
Among them, according to Japanese Patent Publication No. 44-24746, a desired bellows portion is formed in one step by expanding a bead portion using an inner pressure and concurrently causing axial compression using a mold.
According to Japanese Patent Laid-Open No. 59-133021, two core rods with an elastic body interposed therebetween are inserted into a pipe, one core rod is pushed toward the other to deform the elastic body, thereby forming an expanded angle portion in a portion of the pipe, and this expanded portion is shaped so as to conform to a bending mold, whereby a corded pipe is manufactured.
According to Japanese Patent Laid-Open No. 63-207421, a pressure medium is disposed inside a work to apply an inner pressure thereto, mold elements together defining a bead forming recess thereinside are disposed outside the work relatively movably in the axial direction of the work, the work is subjected to plastic deformation so that it is expanded, and the mold elements are caused to approach each other, whereby a beaded pipe is manufactured.
Among the foregoing systems hitherto proposed, in the system of Japanese Patent Publication No. 44-24746, the mold must be prepared so as to meet the length of a pipe and the number of beads of a bellows portion; thus, the machinable pipe length is limited and the mold is expensive.
In the two systems of Japanese Patent Laid-Open Nos. 59-133021 and 63-207421, although the degree of freedom relating to the number of beads in a bellows portion is comparatively large, since the elastic body is disposed as passing through the core, it is difficult to decrease the diameter of the core portion in view of the purpose of ensuring the strength of the core and the volume of the elastic body; thus, the minimum deformable diameter of a pipe is of the order of 16mm.
Further, since a bead forming groove is provided in the mold, the outer diameter and pitch of a bead are limited and the degree of freedom of bead shape is limited. Since a pipe is secured by the mold at two points between which an expanded portion is to be formed, the pipe cannot shift in the axial direction sufficiently during expansion; thus, a top portion of the bead becomes thin, and the elastic body is severely worn because of forced sliding. Further, since the elastic body is caused to expand itself inside the mold, a large tightening force is required for the mold; thus, the elastic body is easily worn because of an unreasonable force being applied.
Furthermore, during successive forming of a bellows portion, the elastic body is compressed by the bellows portion and its edge portion is pinched between the bellows portions; thus, the elastic body is severely worn.
In addition, in the system of Japanese Patent Laid-Open No. 63-207421, since an expanded portion together with the elastic body is compressed in the axial direction, the elastic body tends to be torn off.